test(document): cover TimelineControls and TimelineXAxis branches

TimelineControls: empty render when neither flag is set, reset button
gated on isZoomed, clear button gated on hasSelection, both-on, both
callback wirings.

TimelineXAxis: empty filled → no ticks, populated → ticks render,
omit-year branch when all buckets share a year, show-year branch
across multiple years, length-4 bucket-string fallback.

11 tests across two timeline primitives.

Refs #496.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/TimelineControls.svelte.test.ts

@@ -0,0 +1,84 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import TimelineControls from './TimelineControls.svelte';
+
+afterEach(cleanup);
+
+describe('TimelineControls', () => {
+	it('renders neither button when not zoomed and no selection', async () => {
+		render(TimelineControls, {
+			props: {
+				isZoomed: false,
+				hasSelection: false,
+				onresetzoom: () => {},
+				onclearselection: () => {}
+			}
+		});
+
+		const buttons = document.querySelectorAll('button');
+		expect(buttons.length).toBe(0);
+	});
+
+	it('renders the reset-zoom button when isZoomed is true', async () => {
+		render(TimelineControls, {
+			props: {
+				isZoomed: true,
+				hasSelection: false,
+				onresetzoom: () => {},
+				onclearselection: () => {}
+			}
+		});
+
+		await expect.element(page.getByRole('button', { name: /zur übersicht/i })).toBeVisible();
+	});
+
+	it('renders the clear-selection button when hasSelection is true', async () => {
+		render(TimelineControls, {
+			props: {
+				isZoomed: false,
+				hasSelection: true,
+				onresetzoom: () => {},
+				onclearselection: () => {}
+			}
+		});
+
+		await expect.element(page.getByRole('button', { name: /auswahl zurücksetzen/i })).toBeVisible();
+	});
+
+	it('renders both buttons when both flags are true', async () => {
+		render(TimelineControls, {
+			props: {
+				isZoomed: true,
+				hasSelection: true,
+				onresetzoom: () => {},
+				onclearselection: () => {}
+			}
+		});
+
+		const buttons = document.querySelectorAll('button');
+		expect(buttons.length).toBe(2);
+	});
+
+	it('calls onresetzoom when the reset button is clicked', async () => {
+		const onresetzoom = vi.fn();
+		render(TimelineControls, {
+			props: { isZoomed: true, hasSelection: false, onresetzoom, onclearselection: () => {} }
+		});
+
+		await page.getByRole('button', { name: /zur übersicht/i }).click();
+
+		expect(onresetzoom).toHaveBeenCalledOnce();
+	});
+
+	it('calls onclearselection when the clear button is clicked', async () => {
+		const onclearselection = vi.fn();
+		render(TimelineControls, {
+			props: { isZoomed: false, hasSelection: true, onresetzoom: () => {}, onclearselection }
+		});
+
+		await page.getByRole('button', { name: /auswahl zurücksetzen/i }).click();
+
+		expect(onclearselection).toHaveBeenCalledOnce();
+	});
+});